1. Field of the Invention
The present invention relates to a structure for connecting electric cells (often referred to as an “electric-cell connecting structure”), and more particularly to an improvement in a structure for connecting electric cells, wherein a pair of electric cells, each having a bottomed cylinder of a negative electrode and a seal plate of a positive electrode attached to an open end portion of the bottomed cylinder in such a manner as to seal the open end of the bottomed cylinder, are connected together by way of a connecting bus bar to be welded to a closed end of the bottomed cylinder and to the other seal plates of both electric cells, or by way of a connecting ring.
2. Description of the Related Art
The following connecting structure has already been known such as disclosed in JP-A-2003-162993, wherein, when a pair of electric cells disposed in parallel to each other, are connected together, both end sections of a connecting bus bar are welded to a negative terminal of one of the electric cells and to a positive terminal of both electric cells.
In the above-described connecting structure, a pair of electric cells are connected through use of a connecting bus bar entirely having a uniform thickness. Under the situation in which the connecting bus bar is formed from a metal plate which is thicker than a closed end and a seal plate of a bottomed cylinder of an electric cell, when an external force is applied in such a manner as to pull off the connecting ring from the electric cell, the electric cell will be fractured. Therefore, the connecting bus bar must be thinner than a section of the electric cell to which the connecting ring is welded.
Meanwhile, when the thickness of the entire connecting ring is decreased, connection resistance is increased. In this case, when a large amount of current is charged or discharged from the electric cells, a drop in voltage at the connecting ring becomes large. This may result in inability to take out large electric power or heat development at the connecting bus bar responsible for heating the electric cells, which may shorten the lives of the electric cells.
In order to satisfy such contradictory desires, it is considered to weld a thin metal plate to the electric cells and connect the thin plate metals together by means of welding a thick metal plate, to thus constitute a connecting bus bar. Such a connecting bus bar involves an increase in the number of areas to be welded, which is responsible for a cost hike, and an increase in the weight of the connecting structure, as well as involving an increase in the number of components to be used for constituting the bus bar.
Further, the following connecting structure has been also known such as disclosed in Japanese Patent No. 3312853 or JP-A-2000-268793. In those connecting structures, a pair of electric cells disposed in series with each other are connected together via a connecting ring. This connecting ring integrally includes a large-diameter short cylindrical section, a first flange section, a small-diameter short cylindrical section, and a second flange section. The large-diameter short cylindrical section assumes a cylinder having one open end and is to be fitted over and welded to a surface of an outer periphery of a bottomed cylinder. The first flange section extends radially inward from the other end of the large-diameter short cylindrical section, thereby opposing an outer surface of a closed end of the bottomed cylinder. The small-diameter short cylindrical section, whose one end is connected to an inner periphery of the first flange section, extends in a direction opposite the large-diameter short cylindrical section. The second flange section extends radially inward from the other end of the small-diameter short cylindrical section, and is welded to a seal plate.
In the above-described connecting structure, a pair of electric cells are connected through use of a connecting ring entirely having a uniform thickness. Under the situation in which the connecting ring is formed from a metal plate which is thicker than a closed end and a seal plate of a bottomed cylinder of an electric cell, when an external force is applied in such a manner as to pull off the connecting ring from the electric cell, the electric cell will be fractured. Therefore, the connecting ring must be thinner than a section of the electric cell to which the connecting ring is welded.
Meanwhile, when the thickness of the entire connecting bus bar is decreased, connection resistance is increased. In this case, when a large amount of current is charged or discharged from the electric cells, a drop in voltage at the connecting bus bar becomes large. This may result in inability to take out large electric power or heat development at the connecting ring responsible for heating the electric cells, which may shorten the lives of the electric cells.